1. Field of the Invention
The present invention relates to a device for purifying the exhaust gas of an engine.
2. Description of the Related Art
If a ratio of the total amount of air fed to the intake passage, the combustion chamber, and the exhaust passage upstream of a certain position in the exhaust passage to the total amount of fuel fed to the intake passage, the combustion chamber, and the exhaust passage upstream of the above-mentioned position, is referred to as an air-fuel ratio of the exhaust gas flowing through the certain position, it is well known that an engine, in which the lean air-fuel mixture is burned, has a NO.sub.X absorbent arranged in the exhaust passage, the NO.sub.X absorbent absorbing NO.sub.X therein when the air-fuel ratio of the inflowing exhaust gas is lean, and releasing the absorbed NO.sub.X therefrom when the oxygen concentration in the inflowing exhaust gas becomes lower. In the engine, the air-fuel ratio of the exhaust gas flowing the NO.sub.X absorbent is made rich temporarily to thereby release the absorbed NO.sub.X from the NO.sub.X absorbent and reduce the NO.sub.X.
However, fuel and the lubrication oil contain sulphur containing components, and thus the exhaust gas also contains sulphur containing components, such as SO.sub.X. The NO.sub.X absorbent absorbs the SO.sub.X in the form of SO.sub.4.sup.2-, together with NO.sub.X. However, the SO.sub.X is not released from the NO.sub.X absorbent even when the air-fuel ratio of the inflowing exhaust gas is made rich. Thus, the amount of SO.sub.X absorbed in the NO.sub.X absorbent increases gradually. However, if the SO.sub.X amount in the NO.sub.X absorbent increases, the NO.sub.X absorbing capacity of the NO.sub.X absorbent gradually becomes smaller, and at the last, the NO.sub.X absorbent can hardly absorb NO.sub.X therein.
However, the NO.sub.X absorbent releases the absorbed SO.sub.X therefrom in the form of SO.sub.2, for example, when the oxygen concentration in the inflowing exhaust gas becomes lower with the temperature of the NO.sub.X absorbent being higher than the SO.sub.X releasing temperature thereof. Japanese Unexamined Patent Publication No. 6-88518 discloses an exhaust gas purifying device for an engine in which the air-fuel ratio of the exhaust gas flowing to the NO.sub.X absorbent is made rich temporarily when the temperature of the NO.sub.X absorbent is higher than the SO.sub.X releasing temperature.
The device mentioned above does not include a device for heating the NO.sub.X absorbent, such as an electric heater. Thus, the temperature of the NO.sub.X absorbent is only higher than the SO.sub.X releasing temperature when the engine load is high, for example. However, at the high load engine operation, the flow rate of the exhaust gas flowing through the NO.sub.X absorbent is high, i.e., the contact period between the exhaust gas and the NO.sub.X absorbent is short. However, the SO.sub.X releasing rate of the NO.sub.X absorbent is relatively low, and thus SO.sub.X is not released from the NO.sub.X absorbent sufficiently, even when the air-fuel ratio of the inflowing exhaust gas is made rich with the temperature of the NO.sub.X absorbent is higher than the SO.sub.X releasing temperature, as long as the contact period is short. Namely, the air-fuel ratio of the exhaust gas flowing to the NO.sub.X absorbent must be made rich for a long time to release the SO.sub.X from the NO.sub.X absorbent sufficiently, if the contact period is short.